“If it ain’t broke, don’t fix it,” could be the tagline for Half Sister, Full Love (Et ta soeur), an extremely faithful French adaption of Your Sister’s Sister that switches up the language and nationality but precious little else about the original movie. At the same time, Lynn Shelton’s version was such an enjoyable, cleverly conceived three-hander that director Marion Vernoux was probably right to keep her interference to a minimum, choosing a fine cast to reenact this offbeat story of family, friendship and unrequited love on a remote island setting.

Believe it or not, this is the second Shelton film to be remade in France, with Humpday adapted by actor-director Yvan Attal into 2012’s Do Not Disturb. While that movie took some liberties with the U.S. screenplay, Vernoux mostly plays it by the book, up to and including certain gags and lines of dialogue. Such similarities will pose few problems with Francophone audiences experiencing the scenario for the first time, but it’s hard to imagine the movie getting any real traction stateside.

Vernoux’s best decision came at casting time, with the choice of actresses Virginie Efira (It Boy) and Geraldine Nakache (Playing Dead) to play a pair of half-sisters — the recently separated lesbian, Marie, and her younger, nerdier sibling, Tessa — who reunite at their dad’s secluded country house on a rain-soaked island off the coast of Brittany.

The only snag is that Tessa’s best friend, Pierrick (the excellent Gregoire Ludig), is stuck there as well, unable to get over the recent death of his brother (who was also Tessa’s ex) and screwing everything up when he sleeps with Marie during a first night of drunken, highly expeditious lovemaking. Now there’s a huge secret that separates the three of them, until various twists and quid pro quos make it all come out during the last act.

In Your Sister’s Sister, Shelton did a great job keeping sentiments boiling under the surface until they exploded, and while Vernoux tries to do the same thing here, the French version doesn’t quite sustain all the underlying tension. Part of it may be that the plot is familiar to those who know the first film, but there’s something about the passive-aggressive, Mumblecore-rish ways of Shelton’s world that cannot completely translate to France, where people tend to expose their feelings rather than keeping them under wraps, and where sleeping around (even if it’s with a friend’s sister) is not necessarily so taboo.

There are other, more obvious differences as well, such as the moment where Tessa tries to seduce Pierrick, only to wind up innocently falling asleep in his bed. While the American film gave us a scene where a fully dressed Emily Blunt clumsily flirts with Mark Duplass, the French one has Nakache wearing a low-cut nightie as she gives Ludig a sensual massage that almost turns sexual before the latter gets cold feet.

Comparisons aside, Vernoux has already shown in previous films (including the 2013 senior romance, Bright Days Ahead) that she can direct actors in an emotionally observant manner, and all three castmembers are convincing as 30-somethings trying to work out their issues over the course of a long and stormy weekend.

Nakache is particularly good in a role that skews darker than her usual efforts, while standup comic Ludig is amusingly low-key as an opinionated loser with a heart of gold. The blond, doll-like Efira feels like a bit of a stretch as Marie, but the Belgian actress delivers in most of her scenes, especially the one where we meet her for the first time.

With the rocky Breton coastline subbing in for the San Juan Islands, Half Sister, Full Love — which, by the way, is a terrible international title; the French one is a popular expression that literally translates to “And Your Sister” — has a gloomier atmosphere, with stormy waters constantly rocking the beaches and foiling any escape plans. DP Nicolas Gaurin (Hippocrates) makes strong use of the distinct setting, framing the cast against a backdrop of emotional waves just waiting to break ashore.